I would take a hard look at the fuel distributor. My 87 420sel sat for 1.5 years and I had gremlins until I cleaned the fuel distributor with solvent to remove the varnish that had accumulated in it. I have never done this but I often wondered if running solvent thru the entire system might clear things out.

When a car sits, the fuel deposits varnish in the worst places. Areas like the little orfices and small screens in the fuel system components. I would take things apart piece by piece and clean them with a good solvent that disolves varnish.